1. Field of the Invention
This invention relates to a display control system, which permits access to a memory for display data generation by a main system (for instance an external CPU or a personal computer system) according to a display mode command signal.
2. Description of the Prior Art
FIG. 4 is a block diagram showing a prior art display control system. Referring to the FIGURE, reference numeral 1 designates a display controller (CRTC), 2 a display data memory (VRAM) as first memory means for storing display data, 3 and 4 display data hold circuits (F/F) for temporarily holding display data read out from the VRAM 2, 5 a display data generation memory (CG) as second memory means for storing character pattern data, 6 an image data generation circuit (DGL), 7 a display unit (CRT), 8 a main system side character code hold circuit (F/F), which serves to hold character code data from the main system side and includes a counter for counting bytes of character pattern data corresponding to one character code, 9 a selector for selectively providing an output of either F/F 3 or F/F 8 to the CG 5, 10 a display data generation memory access circuit (DCNT) for permitting access of the CG 5 from the main system side and including an edge trigger flip-flop (F/F) 10a as character pattern data holding means, 11 display data memory access circuit (VRIF), 12 an access mode detection circuit, and 13 a display mode detection circuit.
FIG. 5 shows the image data generation circuit (DGL) 6 in detail. In the FIGURE, designated at 6a is a parallel-to-serial converter for graphics image, 6b an attribute data hold circuit, 6c a parallel-to-serial converter for character pattern, 6d a selector for character image, 6e a border color register, and 6f an image data selector.
FIG. 6 shows the display data generation memory access circuit (DCNT) 10 in detail. In the FIGURE, designated at 10a is the flip-flop for holding the character pattern data noted above, 10b a display data timing detection circuit, and 10c a data take-in timing signal generation circuit.
The operation of the circuit shown in FIG. 4 will now be described. The CRTC 1 provides a signal for controlling the display unit and a control signal for reading out display data of the VRAM 2 and timing signals to the DGL 6 and DCNT 10. From the main system, display data is supplied through the VRIF 11 to the VRAM 2. The CRTC 1 accesses the VRAM 2 and reads out data therefrom, the read-out data being temporarily held in the hold circuits F/F 3 and F/F 4. The subsequent operation is determined depending on display modes. As the display modes, there is a character display mode and a usual graphics display mode other than the character display mode. In the character display mode, character code data is held in the F/F 3, and attribute data in F/F 4. The selector 9 is adapted such that it usually selects an output of the F/F 3. The character code data from the F/F 3 is supplied through the selector 9 to the CG 5 which then provides corresponding character pattern data. This character pattern data and the attribute data from the F/F 4 are supplied to the DGL 6 which produces from these data image data to be provided to the CRT 7. In the case of the graphics display mode, the outputs of the F/Fs 3 and 4 are directly supplied to the DGL 6 which then produces image data supplied to the CRT 7. The DGL 6 receives a signal from the display mode detection circuit 13 to effect switching between character image data and graphics image data.
Referring to FIG. 5, in the graphics display mode the VRAM 2 contains graphic display data. This data is temporarily held in the F/Fs 3 and 4 and then supplied to the parallel-to-serial converter 6a for graphics image. As a result, graphics image data is generated and supplied to the image data selector 6f. In the character display mode, the VRAM 2 contains character display data, and,-as noted before, character code data is held in the F/F 3 and attribute data in the F/F 4. With the output of the F/F 3 the CG 5 is accessed through the selector 9, and read-out character pattern data is held in the parallel-to-serial converter 6c for character pattern. Meanwhile, the attribute data from the F/F 4 is held in the attribute data hold circuit 6b and supplied to the selector 6d for character image. In the selector 6d character color data and background color data in the attribute data are selected according to the output of the parallel-to-serial converter 6c, then the selected data is supplied to the video data selector 6f. The border color register 6e contains data for determining a display color of border area. As shown in FIG. 7, the face area 7a on the screen of the CRT 7 consists of an intrinsic display area 7b and a border area 7c surrounding the display area 7b. The CRTC 1 supplies a signal for selecting either the display area 7b or the border area 7c to switch the image data selector 6f. A signal representing a display mode is supplied from the display mode detection circuit 13, and the video data selector 6f selects either graphics image data or character image data to be provided to the display area 7b.
The operation will now be described in connection with the case when the main system accesses the CG 5. FIG. 8 shows a timing signal DE which is supplied from the CRTC 1 to the timing detection circuit 10b in the DCNT 10. There are two access modes, i.e., an access mode I, in which the main system can access to the CG 5 only during a non-display period t2, and an access mode II, in which the main system can access to the CG 5 at all time, i.e., during either a display period t1 or the non-display period t2. Either access mode is set in the access mode detection circuit 12. In either mode, when character code data is written in the F/F 8 from the main system, the F/F 8 supplies data to the CG 5, whereby corresponding character pattern data is provided. This data is taken in the DCNT 10 and transmitted to the main system under control of a timing signal form the CRTC 1. On the other hand, the main system can transmit character pattern data to the CG 5. When character code data and character pattern data are written from the main system in the F/F 8 and DCNT 10 respectively the character pattern data is transmitted to the CG 5 by a timing signal of the CRTC1. In the character display mode, during the display period the CG 5 should receive the character code data from the F/F 3 through the selector 9 for display data generation and supply corresponding character pattern data to the DGL 6. While there are two different access modes, in the access mode I access of the main system is allowed only during the non-display period t2. The non-display period t2, however, is as short as a fraction of the display period t1. Therefore, the access performance is inferior. In the other access mode II, even during the display period the selector 9 is switched to the side of the F/F 8 in response to an access from the main system, thus improving the access performance. However, in this case data of the main system is transmitted to the DGL 6 as well during the display period, thus resulting in disturbance of the display.
Further detailed description will be given with reference to FIG. 6. The access mode I is the same as the access mode II except for that access is inhibited during the display period. For this reason, the access mode II will be described in detail. FIG. 9 shows a timing signal a which is obtained by dividing a character clock from the CRTC 1 into two. This signal is generated in the data take-in timing signal generation circuit 10c. First, data representing the character display mode is set in the display mode detection circuit 13, and data representing the access mode II is set in the access mode detection circuit 12. Character code data is written from the main system into the F/F 8. When the data take-in timing generation circuit 10c detects the writing of the character code data in the F/F 8 from the rising of the timing signal .alpha.,the selector 9 provides the data from the F/F 8 to the CG 5. The CG 5 accordingly provides corresponding character pattern data to the flip-flop 10a. The character pattern data is held in the flip-flop 10a with the falling of the timing signal .alpha., the held data is then read out by the main system. Meanwhile, with the falling of the timing signal .alpha. a counter in the F/F 8 starts up-counting. Since the character pattern data consists of several bytes per character, after setting of one character code the main system should read out several bytes. These several bytes are set by the counter in the F/F 8. Unless the main system reads out data from the flip-flop 10a, new clock for holding of data in the CG 5 is not provided from the data take-in timing generation circuit 10c to the flip-flop 10a. Thus, data is preserved in the flip-flop 10a, and the data is normally transferred to the main system. When the data is read out from the flip-flop 10a, the main system provides a command (i.e., I/O read signal).
In the case of the graphics display mode, the CG 5 is accessed in the manner as described above. In the case of the graphics display mode, the CG 5 is not directly accessed by graphics image data, so that in the access mode II there is no possibility of disturbance of the display, thus improving the access performance. Again in this case, the timing signal from the CRTC 1 is used as the timing of input of data to the DCNT 10 when reading out the character pattern data after writing of data in the F/F 8 and also as timing of input of data to the CG 5 when writing data.
With the above construction of the prior art display control system, even in the case other than the character display mode, in which the CG 5 is unnecessary, the access to the CG 5 by the main system for reading and writing data is done under control of the timing signal from the CRTC 1, so that it is subject to that timing restriction, thus correspondingly limiting the access performance.